There have been widely used switching power supply units having a coil and a switching element for stepping up an input voltage to a predetermined high output voltage.
Such switching power supply unit has a voltage detection circuit for feeding back the output voltage as a feedback voltage to be compared with a reference voltage. Based on the comparison, the duty ratio and the frequency of the switching element are controlled so as to regulate the output voltage to a desired level. As a backup of the voltage control, the switching power supply unit is provided with an over-voltage protection circuit utilizing the voltage detection circuit to monitor the output voltage such that, should the detected voltage exceeds a predetermined allowable limit, the over-voltage protection circuit execute a protective measure (see for example, Japanese Patent Publication of Examined Application No. H7-50983).
However, such conventional switching power supply unit has a drawback in that the feed back voltage falls to zero when the voltage detection circuit suffers an open-circuit malfunction for some reason, feeding back no voltage. Then a switching-control circuit controlling the switching of the power supply unit makes a determination that the output voltage is insufficient, and acts to raise the output voltage. As a consequence, the output voltage rises towards the maximum allowable level of the switching power supply unit.
Although the output voltage is monitored, such run away of the output voltage happens because the voltage detection circuit for feedback control of the output voltage cannot detect an over-voltage caused by such open-circuit malfunction of the voltage detection circuit.
To circumvent this drawback, an additional voltage detection circuit for detecting an over-voltage may be provided in addition to the voltage detection circuit for feeding back the output voltage. In this case, however, it is necessary to provide an additional terminal for the additional circuit on the control IC for controlling the switching of the power supply unit (the IC referred to as switching control IC). Provision of such additional terminal, however, entails a disadvantage that the switching control IC cannot be fully miniaturized.
A further disadvantage is that an over-voltage condition due to an open-circuit malfunction of a constituent element of the switching power supply unit or breaking of a wire in the unit cannot be detected by a conventional over-voltage detection circuit. Therefore, in that event also the over-voltage can be impressed on various components of the power supply unit, including the switches of the switching control IC and other components.